1. Field of the Invention
The present invention relates to a burner structure applicable to various types of combustion apparatuses such as a pulverized coal boiler.
This application is based on Japanese Patent Application No. 2006-303780, the content of which is incorporated herein by reference.
2. Description of Related Art
Hitherto, boilers fired with a fuel powder, for example, pulverized coal or petroleum coke have been used.
To describe a burner structure employed in a pulverized coal boiler that is fired with pulverized coal, a burner is composed of a pulverized coal-air mixture system provided in the burner center and containing pulverized coal and a primary air, a secondary air system provided around the pulverized coal-air mixture system, and cooling air (tertiary air) systems optionally provided around or above and below the secondary air system.
FIG. 5 is a sectional view showing a pulverized coal burner structure of the related art.
A burner 10 of FIG. 5 is structured such that a secondary air path 12 as a secondary air system is provided around a pulverized coal-air mixture path 11 as a pulverized coal-air mixture system. In addition, a cooling air path 13 as a cooling air (tertiary air) system is provided above the secondary air path 12.
Provided at furnace-side ends of the pulverized coal-air mixture 11 and the secondary air path 12 is a nozzle main body 17 that integrates a pulverized coal nozzle 14 and a secondary air nozzle 15 with a flame holder 16 provided at their tip ends. Further, a cooling air nozzle 18 is attached at the furnace-side end of the cooling air path 13. The cooling air nozzle 18 functions to prevent a falling clinker from the upper part in the furnace from colliding against the burner 10 and to shield a flame radiation heat. In FIG. 5, reference numeral 19 denotes a wind box.
In the thus-structured burner 10, the following combustion method is employed. That is, a fuel and an air are supplied while the total amount of the primary air, the secondary air, and the tertiary air is set smaller than an ideal air amount relative to an amount of pulverized coal loaded to fire the burner as required by the regulations on nitrogen oxides (NOx). In this way, a main combustion zone is kept under a reducing atmosphere. Then, NOx generated upon burning the pulverized coal is reduced, after which an additional air is supplied from an additional air nozzle (not shown) provided on the downstream side of the main combustion zone for oxidation combustion. In this way, combustion is completed. Thus, enough air is supplied around a pulverized coal flow in the main combustion zone.
Further, in the burner 10 of the related art, the nozzle main body 17 is tiltable for controlling a steam temperature or an amount of NOx at the outlet as shown in FIG. 6, but the cooling air nozzle 18 is fixed.
In addition, there is reported another structure that the entire nozzle inclusive of an air flow path corresponding to the above cooling air nozzle 18 is tiltable (see the Publication of the U.S. Pat. No. 6,260,491, for instance).
Recently, an ignition performance has been enhanced year by year along with improvements in the flame holder 16. As a result, materials for the burner 10 are exposed to higher temperatures. On the other hand, if a flow rate of the cooling air supplied to the cooling air nozzle 18 is increased to increase a cooling ability, a combustion temperature lowers and causes an increase in unburned components. Thus, exhaust gas characteristics are deteriorated, so it is necessary to efficiently cool the nozzle main body 17 with a small amount of air.
Moreover, in the burner 10 of the related art, the nozzle main body 17 is only tiltable and the cooling air nozzle 18 is fixed, which causes a problem in that the tilted nozzle main body 17 is exposed to radiation heat.
On the other hand, in the structure where the entire nozzle is tiltable as disclosed in the Publication of U.S. Pat. No. 6,260,491, an air flow rate is determined in accordance with an air-flow-path area ratio, resulting in a problem in that an air flow rate cannot be adjusted during operation.
Further, the part corresponding to the cooling air nozzle 18 does not have a function of protecting the nozzle main body from a falling clinker or radiation heat, which situation might occurs in the case where a fuel powder such as pulverized coal is used. Therefore, this structure is disadvantageous from the viewpoint of ensuring a long component life.
In view of such circumstances, there is an increasing demand for a burner structure that is capable of adjusting an air flow rate and efficiently cooling a nozzle main body with a small amount of air, and takes an efficient countermeasure against a falling clinker or radiation heat.